The present invention relates generally to combine harvesters and, more particularly, to the cleaning apparatus thereof with a view to improving substantially its grain cleaning characteristics especially when operating in grain crops with tough straw. The invention is particularly advantageous when used in combination with a so-called single-acting cleaning shoe, i.e. a cleaning apparatus comprising at least a pair of juxtaposed cleaning sieves which are oscillated back-and-forth in unison.
It has been experienced that in such cleaning apparatuses, straw pieces tend to bridge and get stuck between the juxtaposed sieves. This phenomenon lies at the origin of sieve plugging resulting in sieve overloading, reduced cleaning efficiency, reduced cleaning capacity, a dirty grain sample (i.e. pieces of straw in the graintank) and increased grain losses; all of which, of course, are not acceptable.
While this tendency for straw pieces to get stuck between juxtaposed sieves is not new, it has become critical and problematic only during recent years. The explanation therefore apparently can be found in the newly applied plant growing techniques and the introduction of new crop varieties in grain farming. With these new plant growing techniques are meant the widespread combined usage of increased nitrogen fertilization and growth regulators. These new techniques together with said new crop varieties result in the crop having much tougher straw at the harvesting time due to the fact that, on the one hand, the crop ears are ripe well before the crop stems and, on the other hand, harvesting is started as soon as the ears have a sufficient degree of ripeness.
The foregoing problem, in most circumstances, may be overcome by the provision of a so-called double-acting cleaning shoe in the combine harvester. In such cleaning shoes the sieves are oscillated in opposite directions whereby straw particles have a lesser chance to get stuck therebetween. However, such a double-acting cleaning apparatus is more complicated in design and more expensive to manufacture. Accordingly, it would be desireable to provide a solution to the straw briding problem usable in conjunction with a single-acting cleaning shoe.